1. Field of the Invention
The present invention relates to a Faraday rotator which constitutes an optical isolator or an optical circulator for use in an optical fiber communication system, an optical recording system, an optical measurement system or the like.
2. Description of the Related Art
In an optical fiber communication system having a semiconductor laser as a light source, in particular, an optical system based on a high speed digital transmission or an analog direct modulation mode, if reflected light from optical connector junctions, optical circuit components and the like which are used in an optical fiber circuit returns to the semiconductor laser or an optical amplifier, it becomes difficult to maintain high quality transmission due to degradation of frequency characteristics or generation of noises. An optical isolator is used for the purpose of removing the reflected light.
As shown in FIG. 8, a conventional optical isolator is constituted by a polarizer 6 and an analyzer 5 both of which transmit only light of a specific plane of polarization, a Faraday rotator 4 composed of a light transmissible substrate 9 and a laminate film 3 formed thereon and adapted to rotate the plane of polarization of light by 46 degrees, and a permanent magnet (not shown) for applying a magnetic field to the Faraday rotator. Among the constituent members of the optical isolator, the Faraday rotator 4 has a primary influence on the performance of the optical isolator. It is important for the Faraday rotator 4 to have a small element length required for rotating the plane of polarization by 45 degrees and a large light transmittance.
Up to now, the Faraday rotator has been made of an yttrium iron garnet (YIG) bulk single crystal (about 2 mm in thickness), or of a bismuth-substituted rare earth iron garnet (BiYIG) thick film single crystal (several hundred xcexcm in thickness) in which part of yttrium is substituted with bismuth having a large magneto-optical performance index. Recently, the BiYIG thick film single crystal is employed in many cases because it is advantageous in downsizing the optical isolator.
Further, in recent years, the magneto-optical component (Faraday rotator) made of one-dimensional magneto-photonic crystal which causes the enhancement of the magneto-optical effect due to the localization of light is proposed. Though the above-mentioned magneto-optical component is of polycrystal with a thickness of several xcexcm, a large Faraday rotation angle can be obtained.
With regard to the one-dimensional magneto-photonic crystal, various structures are proposed. Among them, up to now, the Fabry-Perot resonator structure, in which a magneto-optical thin film is sandwiched between two periodic dielectric multilayer films, obtains a large Faraday rotation angle with the smallest number of layers.
While a (Ta2O5/SiO2) system is generally employed as the dielectric for constituting a dielectric multilayer film serving as a reflecting mirror of the Fabry-Perot resonator, a (Si/SiO2) system has also been proposed which can obtain a large Faraday rotation angle with a smaller number of layers than the (Ta2O5/SiO2) system (for example, Japanese Patent Laid-open No. P2002-49006 A). The film thickness of each dielectric needs to be designed such that its optical length (optical path lengthxc3x97refractive index) is equal to xcex/4 (xcex: a wavelength of light). In addition, it is general that the optical length of an irregular layer (defect layer) composed of a magneto-optical thin film which causes localization of light is set equal to kxcex/2 (xcex: a positive integer).
However, the Faraday rotator with the Fabry-Perot resonator structure has a trade-off relationship between a Faraday rotation angle and a light transmittance, and it has been found that the transmittance is reduced to approximately 50%, if the Faraday rotation angle is increased up to 45 degrees (or xe2x88x9245 degrees), which is the requirement of the optical isolator. 50% of light, which is not transmitted, is reflected in the multilayer film and returns to a light source. This is extremely adverse because the optical isolator is used for the purpose of blocking return light. Further, a reduction in the light emission amount means that a transmittable distance of light is shortened, which makes it difficult to construct a light transmission system.
The present inventors have found that the structure in which two Fabry-Perot resonators sandwich a dielectric thin film having a low refractive index and having an optical length of xcex/4+mxcex/2 (m: 0 or a positive integer) (hereinafter referred to as D.H.W. (double half wave)) is effective in improving the transmittance. The details are disclosed in Japanese Patent Application No. 2000-274936 that was filed on Sep. 11, 2000 and has not been open to the public. With a film structure of (Ta2O5/SiO2)8/BiYIG/(SiO2/Ta2O5/SiO2)8/BiYIG/(SiO2/Ta2O5)8, a Faraday rotation angle of 45 degrees, a transmittance of 99.9% or more and a reflectance of 0.1% or less can be obtained, which raises no problem in practical use.
Further, the present inventors have found that the transmittance and the Faraday rotation angle are compatible with each other also when the dielectric thin film and the magneto-optical thin film have a structure of [L/(H/L)Xxe2x88x921/H/M/H/(H/L)Xxe2x88x921]N/L (hereinafter referred to as S.B.P (square band pass) structure), where L indicates a dielectric thin film with a low refractive index, H indicates a dielectric thin film with a high refractive index, M indicates a magneto-optical thin film, X is the number of repetition of a bilayer (H/L), and N indicates the number of repetition of a base periodic structure. The details are described in Japanese Patent Application No. 2000-338973 that was filed on Nov. 7, 2000 and has not been open to the public. In the above formula, when L=SiO2, H=Ta2O5, M=BiYIG, X=7 and N=3, a transmittance of 100% and a Faraday rotation angle of 45 degrees are obtained.
It has been found that the transmittance and the Faraday rotation angle can be compatible with each other with the above-mentioned multilayer film structures. However, the structures have as many as 67 and 85 layers in total, respectively, including two and more magneto-optical thin films (BiYIG) which require heat treatment. Thus, a process of lamination is complicated, which puts a limitation on a reduction in manufacturing cost. Further, there is a problem that light returning to the light source is generated if the component is manufactured with an optical length deviating from a designed value.
Under the above-mentioned circumstances, the present inventors have proposed and examined a Faraday rotator with a reflection structure as shown in FIG. 9 (Japanese Patent Application No. 2001-100925 that was filed on Mar. 30, 2001 and has not been open to the public), in which a metal reflective film 2 is formed on a substrate 1, then a first periodic dielectric multilayer film 3p, a magneto-optical thin film 3m and a second periodic dielectric multilayer film 3pxe2x80x2 that has the same number of bilayers as the first periodic dielectric multilayer film are formed on the metal reflective film 2, and in which light, which is made incident at a predetermined inclination angle with respect to a direction normal to the film on a side on which the metal reflective film 2 is not formed, is reflected at the metal reflective film 2. In the above-mentioned Faraday rotator with the reflection structure, a large Faraday rotation angle can be obtained with a small number of layers, and also light returning to the light source can be completely controlled. However, a problem emerged that incident light is somewhat absorbed by the metal reflective film, and thus, outgoing light ratio ((amount of light emitted from the Faraday rotator/amount of light made incident on the Faraday rotator)xc3x97100) % is reduced.
The present invention has been made in the light of the foregoing, and it is therefore an object of the present invention to provide a Faraday rotator of multilayer film type, which has a reflection structure that enables a drastic reduction in the number of layers and which can obtain a Faraday rotation angle without suffering absorption loss of light.
In order to achieve the above-mentioned object, according to the present invention, in a Faraday rotator with a reflection structure, two periodic dielectric multilayer films have a number of layer different from each other such that the number of layers of one periodic dielectric multilayer film that is in contact with a metal reflective film is larger than that of the other periodic dielectric multilayer film, whereby outgoing light ratio is improved to 100% while a Faraday rotation angle is kept unchanged.
According to a first aspect of the present invention, in a Faraday rotator, which comprises a first periodic dielectric multilayer film structured such that two kinds of dielectric thin films each having a refractive index different from the other are alternately laminated with regularity in each thickness thereof, a second periodic dielectric multilayer film structured such that two kinds of dielectric thin films each having a refractive index different from the other are alternately laminated with regularity in each thickness thereof, a magneto-optical thin film and a metal reflective film, and in which the magneto-optical thin film is sandwiched between the first periodic dielectric multilayer film and the second periodic dielectric multilayer film, the number of layers of the first periodic dielectric multilayer film is larger than that of the second periodic dielectric multilayer film, and the metal reflective film is formed on a side of the first periodic dielectric multilayer film opposite to a side in contact with the magneto-optical thin film.
According to a second aspect of the present invention, in the Faraday rotator of the first aspect, the first and second periodic dielectric multilayer films are each structured such that a dielectric thin film of the two kinds, which has a lower refractive index and constitutes the first or second periodic dielectric multilayer film, is in contact with the magneto-optical thin film, and a dielectric thin film with a low refractive index is provided between the metal reflective film and the first periodic dielectric multilayer film.
According to a third aspect of the present invention, in the Faraday rotator of the second aspect, an optical length of the dielectric thin film with a low refractive index is xcex/4 (xcex: a wavelength of light).
According to a fourth aspect of the present invention, in the Faraday rotator of the first aspect, at least one of the layers constituting the first or second periodic dielectric multilayer film is substituted by a magneto-optical thin film.